A thought experiment Imagine ants living on a merry goround turning at relativistic speed, which is their two-dimensional world. From measurements on small circles they are thoroughly familiar with the number 2. When they measure the circumference of their world, and divide it by the diameter, they expect to calculate the number π 2 = 3.141 59. We see the merry-go-round turning at relativistic speed. From our point of view, the ants’ measuring rods on the circumference are experiencing length contraction in the tangential direction; hence the ants will need some extra rods to fill that entire distance. The rods measuring the diameter, however, do not contract, because their motion is perpendicular to their lengths. As a result, the computed ratio does not agree with the number 2. If you were an ant, you would say that the rest of the universe is spinning in circles, and your disk is stationary. What possible explanation can you then give for the discrepancy, in light of the general theory of relativity?